Burner for spectroscopic use



Jun 23, 1910 J. F. RENDWA 3,516,17

BURNER FOR SPECTROSCOPIC USE Filed Au 1, 1968 52 1e G02 52 4?; e4 70 a03d 44 62 INVENTOR 50 as v 24 20 14 68 P W-m ATTORNEYS United StatesPatent US. Cl. 431-126 11 Claims ABSTRACT OF THE DISCLOSURE the flowstream upstream of the restriction. The nature of the restriction issuch that the gas velocity through it exceeds the burning velocity ofthe combustion gas. The volume of the flow passage between therestricted portion and the outlet slot is maintained small to reduce theeffects of flashback.

This invention relates to a burner useful with spectroscopic instrumentsand, more particularly, to a burner useful in atomic absorptionspectroscopy.

BACKGROUND OF THE INVENTION Many different types of analyses utilize aflame to impart energy to a sample to the point where the sample eitheremits or absorbs characteristic radiation. Alternatively, flames havebeen used as light sources to provide characteristic monochromaticradiation. Regardless of the particular usage, most of these systemshave certain features in common. These common features include thenebulization of a sample prior to introduction into the flame.

Whether the sample is sprayed directly into the flame or is mixed with acombustible gas prior to the flame, one of the problems encounteredhas-been the problem of thorough atomization or nebulization of thesample- Even with the best nebulizing techniques, the nebulizedparticles often either tend to recombine or are not thoroughly nebulizedto begin with. In either event, the resulting larger particles tend toproduce discontinuous reaction bursts. These discontinuous reaction:bursts distort the analysis.

There is also the ever present problem of the so-called flashback whichrefers to an explosion of the combustible gas within the confines of theburner itself. As a minimum, this explosion extinguishes the flame. Inthe extreme, it can damage the burner and even nearby laboratoryworkers. The combustible gas often is rn'nred with the combustion gas atthe very last minute to reduce the flashback hazard. This results inpoor mixing and degraded combustion. Some of these burners have arelatively large fuel consumption, are limited in the fuels and oxidantsthat can be used, are both physically and opti- 3,516,771 Patented June23, 1970 cally noisy, or provide a relatively short optical path. Alonger optical path often is desirable in the case of atomic absorptionspectroscopy. Many of these prior art burners have the additionalproblem of being easily clogged and relatively diflicult to clean.

Accordingly, it is an object of this invention to obviate many of thedisadvantages of the prior art burners.

Another object of this invention is to provide an improved burner havinga reduced flashback tendency, and which is relatively easy to clean.

SUMMARY OF THE INVENTION A preferred embodiment of the inventionintroduces a stream of nebulized sample droplets entrained in acombustion supporting gas into a flow passage leading to an exit slot ina burner. A portion of the flow passage immediately upstream of the exitslot is restricted to the degree that the flow velocity of the gas anddroplet mixture exceeds the burning velocity of the combustion gas used.Immediately upstream of the restricted portion, the combustion gas isintroduced radially inward into the flow stream from points about theperiphery of the flow passage. The peripheral introduction enhances theuniformity of the resulting combustion mixture. Furthermore, the volumeof the flow passage between the restricted portion and the exit slot hasa reduced volume to limit the effects of flashback. In one embodiment ofthis invention, a nebulizing nozzle directs a stream of dropletsentrained in a combustion supporting gas horizontally into a mixingchamber. A baffle is disposed in the stream of droplets to incurturbulence of the stream and permit larger droplets to drop to thebottom of the mixing chamber. The burners flow passage communicatesvertically with the mixing chamber such that the finely nebulizeddroplets move upwardly from the mixing chamber to the exit slot of theburner.

DESCRIPTION OF THE DRAWINGS The novel features of this invention whichare considered characteristic are set forth with particularity in theappended claims. The invention, itself, however, both as to itsorganization and method of operation, as well as additional objects andadvantages thereof, will best be understood from the followingdescription when read in connection with the accompanying drawings, inwhich:

' FIG. 1 is a pictorial view of the burner of this invention includingthe mixing chamber for a combustion sup? porting gas and nebulizedsample and the elements forming a flow passage in which the combustiblegas is com} bined with the combustion supporting gas and nebulizedsample;

FIG. 2 is a plan view of the upper portion of the burner forming theflow passage showing the details of the exit slot;

FIG. 3 is a sectional View taken through the section line 33 of theupper portion of the burner illustrated in FIG. 1; I

FIG. 4 is a bottom plan view of the upper portion of the burner; and v pFIG. 5 is a cross-sectional elevation view of the entire burnerincluding the burner head as Well as the nebulizing chamber.

3 7 DESCRIPTION OF THE PREFERRED EMBODIMENT The pictorial drawing ofFIG. 1 illustrates quite clearly a burner which includes a nebulizingchamber 10 and a head portion 12 in which the combustible gas is mixedwith a combustion supporting gas and the sample droplets. The details ofthe nebulizing chamber 10 are perhaps seen most clearly in thecross-sectional view of FIG. 5. The nebulizing chamber 10 includes ahollow cylindrical member 14 which has disc shaped end caps 16 and 18,respectively, which close either end of the cylinder 14. Each of the endcaps 16 and 18, respectively, may be flanged as at 20 to aid in theirproper positioning within the cylinder 14. The end caps 16 and 18 may bewelded in position as desired. In some instances a press fit will besuflicient to provide the desired seal. The lefthand end cap (in thedrawing) 18 has a central bore 22 formed along the axis of the cylinder14 to accommodate a nebulizer 24.

The nebulizer 24 is made up of two parts, a base member 30 which has acentral bore 32 and a counterbore 34. A sample input tube 36 ispositioned in the central bore 32. The base member 30 has a secondcounterbore 38 in which a nebulizer head 40 is heated and welded ifdesired. The nebulizer head 40 is in the form of a hollow cylinder whichis seated within the second counterbore 38 and has a central bore 42 ofroughly the same diameter as the counterbore 34 of the base member 30.Set screws 44, positioned radially at 120 angles about the nebulizerhead 44, serve to position the sample input tube 36 such that it islocated directly in the center of the orifice 46 of the nebulizer head.The end portion of the central bore 42 is tapered to form the orifice46. An annular peripheral groove 50 is formed in the exterior of thenebulizer head 40 to accommodate a sealing O-ring 52 which seals theinterior of the nebulizer chamber 10. Finally, the base member 30 isbored radially to accommodate a conduit 54 which transports a combustionsupporting gas into the counterbore 34 for passage about the sampleinput tube 36 and through the orifice 46 to nebulize the fluid sample aswill be described.

The highthand (in the drawing) end cap 16 also has a central axial bore60 which is threaded to accommodate a threaded spoiler adjusting rod 62.A spherical spoiler or bafile 64 is secured to the end of the spoileradjusting rod 62. A knurled knob 66 permits the spoiler adjusting rod 62to be rotated and thereby adjust the axial position of the spoiler 64with respect to the orifice 46 of the nebulizer head 40. A drain tube 68is welded in the bottom portion of the cylinder 14. The drain tube 68 isconnected to a drain trap (not shown).

- The upper side of the cylinder 14 has a radial opening 70 formedtherein. A hollow cylindrical neck 72, cut to conform to the cylindricalsurface of the cylinder 14, is welded to the outer portion of thecylinder 42 at the orifice 70. A tubular tapered neck portion 74 isadapted to be inserted into the chamber neck 72.. An internal annulargroove 76 formed in the chamber neck 72 is adapted to accommodate anO-ring 78 which provides a seal for the neck portion 74. The burner head12 is welded to the top of the neck 74. The tapered neck portion 74 isflared as may be seen in FIGS. 1 and 3 to fit within a slot in the lowerportion of the burner head 12 and thereby provide a flow passage for thenebulized sample, as will be described, upwardly through the burner head12 to an exit slot 80.

The burner head 12 consists essentially of three parts a burner base 84and a pair of side portions 86. The side portions 86 are removablysecured together as by screws or bolts 88 (FIG. 3) and in turn areattached to the burner base 84 by an additional set of screws 90 (FIG.2). The burner base has a central, vertically disposed slot or flowpassage 94 formed therein that is generally rectangular incross-section. The abutting side members 86 together form a flow path 96in the form of a slot-type channel. The flow passage 94 narrows in theupper portion of the burner head to end in the exit slot where the flame81 forms and is flared at the lower portion to have the samecross-sectional area and shape as the slot 94.

A sealing gasket 102 may be positioned between the side members 86 andthe burner base 84 to provide a seal and also to raise the side members86 away from the base 84 sufliciently to provide a small continuousslit-like opening 104 between the slot-like flow path 94 in the burnerbase and an annular trough 106, seen most clearly in the illustration ofFIG. 4, which is formed in the upper portion of the burner base 84. Theslit-like opening is a continuous annular slit that permits thecombustible gas to be introduced radially into and from the entireperiphery of the flow stream of combustion supporting gas and nebulizedsample. The thickness of the slit is maintained small so that thecombustible gas can flow at a greater velocity than the flow stream.This enhances thorough mixing. A conduit 108, connected from a source ofcombustible gas such as hydrogen or acetylene is Welded in a bore 110 tocommunicate with the closed trough 106 and supply a combustible gasthereto. The cross-sectional area of the trough 106 must be suflicientto permit the combustible gas to completely fill the slits 104 such thatthe combustible gas is entering the flow stream peripherally at allpoints substantially simultaneously and at the same rate.

To further aid in mixing the combustible gas with the flow stream amixing rod 98 is positioned along the length of the flared portion ofthe flow path 96. The mixing rod 98 is in the form of a cylindrical rodhaving a pair of end studs which are inserted into the notches formed inthe side portions 86 and most clearly seen in the cross-sectional viewof FIG. 3. The mixing rod 98 is formed to have a diameter slightly lessthan the smaller dimension of the rectangular cross-section of the flowpassage 96 at the point where the mixing rod is positioned. Thisprovides a restricted portion through which the combustible gas alongwith the nebulized sample and combustion supporting gas must flowthereby to increase the velocity of the combustible gas to a pointexceeding that of the burning velocity of the gas. This aids inpreventing flashback from the exit slot from propagating beyond themixing rod 98. For this purpose several sizes of mixing rods may besupplied to accommodate fuels having different burning rates.

In operation a combustion supporting gas such as air or oxygen is passedthrough the orifice 46 of the nebulizer at a relatively high velocity.The velocity must be sufiicient such that it will nebulize and entrainsample supplied to the sample input tube 36 and direct a nebulized oratomized stream of the sample and the combustion supporting gas into thenebulizer chamber 10. This stream of nebulized sample is directedagainst the spoiler 64 whose axial position within the cylinder 14 maybe adjusted by rotation of the knob 66. The spoiler functions to createturbulence within the stream such that heavy droplets have a betterchance of combining with each other at an early stage and dropping tothe bottom of the chamber 14 from which they may be drained through thedrain tube 68 to a trap.

Those properly entrained droplets which do not fall to the floor areswept upwardly through the neck 74 into the burner head 12. As the flowstream passes through the slot-type passage 94 in the lower portion ofthe burner head 12 and reaches the top of the burner base, it is thencombined with a combustible gas fed through the supply tube 108 to theenclosed trough 106. The enclosed trough 106 completely surrounds theslot-type passage 94 through which the flow stream 'of nebulized sampleis passing such that combustible gas is entrained by and mixed with thenebulized sample at all peripheral points of the passage 94simultaneously. The slit must be narrow so that the velocity of thecombustible gas flow therethrough is greater than the velocity of theflow stream through the passage 94. This enhances mixing.

Immediately thereafter, the mixture of combustible gas, nebulizedsample, and combustion supporting gas passes around the mixing rod 98.In doing so the restriction in the flow passage afforded by the mixingrod 98 causes the velocity of the mixture to exceed that of the burningvelocity of the gas employed. The diameter of the mixing rod is selectedso that this condition prevails. There is a turbulence area immediatelydownstream of the mixing rod where the flow streams, split or divided bythe mixing rod, recombine. This turbulence enhances thorough mixing ofthe gases and nebulized sample making up the flow stream immediatelyprior to the exit slot where combustion occurs. From this point thegases continue along upwardly through the flow path to the exit slot 80whence the flame is formed to cover the entire length of the exit slot.This permits a long flame section which is particularly useful in atomicabsorption spectroscopy.

The particular advantages of this invention are many. For one, in theevent there is a flashback from the exit slot 80, the flame cannottravel beyond the mixing rod 98 since the velocity of the passage aboutthe mixing rod exceeds the burning velocity of the flame. By maintainingthe volume of the flow passage 96 between the exit slot 80 and themixing rod 98 relatively small, the magnitude of the explosion caused bythe flashback is reduced.

Secondly, the additional mixing afforded by the mixing rod insuresuniform mixing of the combustible gas with the nebulized sample andcombustion supporting gas. This mixing is enhanced by the peripheralintroduction of the combustible gas to the flowing stream. Additionally,the horizontal direction of the initial nebulized sample stream greatlyfacilitates the dropping out of the heavier, poorly nebulized sampledroplets. Having been initially horizontally directed the selecteddroplets must be sufliciently light to turn a 90 angle and be directedupwardly through the neck 74 into the burner head 12. This provides avery thorough selection basis for selecting only those droplets whichare thoroughly nebulized before they are introduced into the flame. Thisprevents the uneven flashing which often occurs in a spectral burner ofthis type.

The construction of the burner permits its ready disassembly. This readydisassambly is particularly enhanced by the use of the O-rings. Thedisassembly is necessary to permit thorough cleaning of the burner andthe burner head 12 itself is completely disassembled by the simpleremoval of six screws 88 and 90. All of the materials em ployed in thisburner preferably are stainless steel or similar material which cannotonly withstand heat but are not chemically reactive with the sample.Clogging of the burner is unlikely. The burner has a low fuelconsumption rate, can be used with a Wide variety of fuels, and isrelatively quiet both optically and physically.

While only a few gases are specifically described, it is to beunderstood that virtually any of the known combustible and combustiongases may be used.

There has thus been described a relatively unique spectral flame burnerwhich is easy to disassemble, is subject to reduced flashback damage andinsures thorough mixing of the fuel with the combustion supporting gas.

It will be obvious that various modifications may be made in theapparatus and in the manner of operating it. It is intended to coversuch modifications and changes as would occur to those skilled in theart as far as the following claims permit and as far as consistent withthe state of the prior art.

What is claimed is:

1. A burner apparatus comprising:

a droplet separating chamber,

a nebulizing nozzle assembly directing a stream of droplets entrained ina combustion supporting gas horizontally into said chamber,

means including a baffle disposed in said chamber in the path of saidstream for deflecting said stream away from the horizontal,

a burner assembly,

a vertically disposed conduit communicating with said chamber forpassing said droplet stream to said burner assembly,

said burner assembly having a body portion defining a flow passage andan exit slot for said droplet stream,

mixing means in said body portion for mixing a combustion gas with saiddroplet stream, and

means downstream of said mixing means for restricting said flow passage,whereby the flow velocity of said droplet stream and said combustiblegas through the restricted portion exceeds the burning velocity of saidcombustible gas.

2. An apparatus according to claim 1 which also includes means foradjusting the position of said baffle along the axis of said stream.

3. An apparatus according to claim 1 wherein said mixing means formixing a combustible gas with said flow stream includes means defining aperipheral slit opening inwardly into said flow passage.

4. An apparatus according to claim 3 wherein said slit is oriented toopen radially into said flow passage and dimensioned to increase theflow rate of said combustible gas over that of said droplet stream.

5. An apparatus according to claim 3 wherein said slit extendscontinuously about said flow passage.

6. An apparatus according to claim 1 wherein said flow passage issubstantially rectangular in cross section.

7. An apparatus according to claim 6 wherein said restricting meansincludes a partition positioned in said flow passage adapted to dividesaid flow passage, whereby said droplet stream with said combustible gasis split into two streams and again recombined prior to passage throughsaid exit slot.

8. An apparatus according to claim 6 wherein said restricting meansincludes a partition positioned in said flow passage adapted to dividesaid flow passage, Whereby said droplet stream with said combustion gasis split into two streams and again recombined prior to passage throughsaid exit slot.

9. An apparatus according to claim 8 wherein said partition is acylindrical rod.

10. An apparatus according to claim 7 wherein said mixing means formixing a combustible gas with said flow stream includes means defining aperipheral slit opening inwardly into said flow passage.

11. A burner apparatus comprising:

a body portion defining a flow passage having a generally rectangularcross section, and an exit slot, said body portion including:

a base member defining the upstream portion of said flow passage, saidbase member having a planar surface on its downstream side and anannular trough in said planar surface about said flow passage, and

a pair of upper members having mating surfaces defining the downstreamportion of said flow passage and said exit slot, said upper membersbeing secured to said planar surface of said base member, thereby tocomplete said flow passage and define with said planar surface aperipheral slit continuously connecting said trough with said flowpassage;

means for introducting a stream of nebulized sample droplets entrainedin a combustion supporting gas into said flow passage;

means for introducing combustible gas into said trough for passagethrough said peripheral slit to mix combustible gas with said stream ofnebulized sample droplets; and

rod means positioned in said flow passage between said References CitedUNITED STATES PATENTS Axdorfer 431354 Graham 43 l-346 Vallely 431-354Heylman et al.

v 8 1,785,804 12/1930 Adams 431 4 2,714,833 8/1955 Gilbert 431 4 XRFOREIGN PATENTS 140,576 4/1920 Great Britain. 5 367,747 8/1930 GreatBritain.

FREDERICK L. MATTESON, JR., Primary Examiner R. A. DUA, AssistantExaminer US. Cl. X.R.

